Hydrogen storage alloys have been manufactured in a drastically increasing amount since the alloys were used for anodes of batteries. The hydrogen storage alloys presently used for batteries are mostly AB5, type alloys, which contain La or misch metal, a mixture of light rare earth elements, on the A-site, and Ni on the B-site, which is partially substituted by Co, Mn, Al, or the like element. The amount of hydrogen that such AB5 type alloys are capable of absorbing and desorbing under the hydrogen pressure of 0.01 to 4 MPa (defined as effective hydrogen storage capacity) is at most 1.2 wt %. When the alloy having such an effective hydrogen storage capacity is used for producing a hydrogen storage tank mounted on a fuel cell electric vehicle, which is under active development, the required amount of the alloy weighs too much. In order to overcome this drawback, Cr—Ti—V alloys principally of a body-centered cubic crystal structure (BCC structure) are recently under development as a different line of hydrogen storage alloys, which have an effective hydrogen storage capacity of over 2 wt %.
Cr—Ti—V alloys have excellent properties, but require higher temperatures in their production for melting the essential elements V and Cr for alloying, which have melting points of 1910° C. and 1863° C., respectively. In addition, Ti, which also has a melting point of as high as 1670° C., is an active element, and thus requires careful selection of a crucible in which it is melted. That is, if a Cr—Ti—V alloy is melted in a crucible made of a metal oxide such as alumina, magnesia, or zirconia, Ti reacts with the main component of the crucible to corrode the walls of the crucible, which are cracked and become unusable for melting. Thus in practice, alloys containing active Ti with a high melting point are merely under pilot production by arc melting in a water-cooled copper crucible. However, in melting an alloy in a water-cooled copper crucible, the portion of the alloy melt that is in contact with the crucible is not melted, which leads to segregation and poor thermal efficiency. Thus this method is not suitable for mass-production.
It is known that the reaction of Ti with an oxide crucible becomes severer as the temperature increases. Thus, the burden to the crucible may be alleviated if the melting point of the master alloy is lowered. In this regard, JP-9-49034-A discloses a method for producing a BCC hydrogen storage alloy containing at least V and Ni, in which a V—Ni, Ti—V, or Fe—V alloy produced by thermit reduction is used as a starting material. JP-2000-96160-A discloses a method for producing a material for a V-containing hydrogen storage alloy having the Al content of less than 1 wt % by thermit reduction of an alloy material containing V-oxide and optionally Ni, Fe, Cu, Co, Mn, Cr, Nb, Ta, and the like element, using Al or an Al alloy as a reducing agent. JP-11-106847-A discloses a method for reducing the oxygen content of a V-containing hydrogen storage alloy produced by thermit reduction, wherein the alloy is melted under heating with a deoxidizing agent such as Ca, Mg, rare earth elements, or the like, for improving the properties of the alloy.
As can be seen from these methods, production methods are being developed which employ alloys of V and a transition metal as a master alloy of a hydrogen storage alloy, instead of metal V, which is high in both melting point and cost. Also the thermit reduction is recognized as a favorable method for mass-production, compared to the above-mentioned arc melting in a water-cooled copper crucible.
In producing a Cr—Ti—V hydrogen storage alloy, however, when a Ti oxide and a V oxide are reduced with metal Al by thermit reaction for obtaining a Ti—V alloy in accordance with the method disclosed in JP-9-49034-A, the Ti oxide cannot be reduced sufficiently with Al, so that a large amount of Al remains in the resulting alloy, in particular in producing an alloy having the Ti content of not lower than 10 at %. Such an alloy containing an excess amount of Al has a remarkably low hydrogen storage capacity, and cannot achieve the effective hydrogen storage capacity of not lower than 2 wt %.
In order to overcome this problem, JP-2000-96160-A discloses to use, as a reducing agent, 85 to 99% of the theoretical amount of Al required for reducing all the oxides in the alloy material, in order to reduce the Al content in the resulting alloy to not higher than 1 wt %. However, when this method is applied to production of a Cr—Ti—V alloy, the Cr—V alloy produced by thermit reduction has to be remelted before Ti is added. For remelting, the Cr—V alloy has to be heated to as high as not lower than 1750° C., which severe temperature condition remarkably impairs the life of the crucible. Further in this method, wherein the amount of the reducing agent Al is lowered below its theoretical amount for lowering the residual Al content to not higher than 1 wt %, enough heat is not generated in the reducing reaction when the Al content is not higher than 95%, in particular not higher than 90% of the theoretical amount. This makes it difficult to maintain the alloy melt at a required high temperature for a required duration, which results in insufficient separation of the oxides from the alloy melt by floating.
In the above-described prior art, no method has been established that realizes intensive production of a high-purity, multi-element alloy containing V, utilizing thermit reduction, without remarkably impairing the service life of an expensive crucible. In particular for production of the alloy also containing Ti as an essential element, V produced by thermit reduction or an alloy of V and other alloy elements than Ti has to be the master alloy, to which metal Ti is added, and remelted at a high temperature. This inevitably involves additional consumption of thermal energy and wearing of the crucible.